Method and apparatus for applying metal coatings



Jan. 10, 1939. c. H. WALKER ET AL v METHOD AND APPARATUS FOR APPLYINGMETAL COATINGS Filed March 13, 1935 WWW/247% mum I p s m TM T O o t S mm OA HWT Hr a E T we Patented Jan. 10, 1939 UNITED ,STATES PATENT OFFICEMETHOD AND APPARATUS FOR APPLYING METAL COATINGS Application March 13,1935, Serial No. 10,942 In Great Britain April 13, 1934 Claims.

The present invention relates to an improved method and apparatus forapplying metal coatin 's to surfaces by evaporation distillation ofmetals in a vacuum to thereby obtain for exam- 5 pie astronomical andother reflecting mirrors, electrical condenser sheets such as metalcoated paper, which may also be used for low current fuses, and coatedtextile materials.

The coating of surfaces by evaporation or distillation of metals invacuum has heretofore been difficult and uncertain owing to therelatively high temperature at which the evaporation took place andowing to the difliculty of obtaining a receptacle or holder for themetal to be evaporated, since by reason of alloying or other chemicalaction between the metal to be evaporated and said receptacle or holderthe latter tends to rot and become unserviceable. It has been diflicultto regulate the rate of evaporation ordistillation and the results havebeen both poor and variable, at least from commercial considerations.When an alloy is formed, as is usually the case, between the receptacleor holder and the metal to be evaporated or distilled, the alloygenerally has a melting point considerably lower than the evaporation ordistillation temperature of the coating metal and even with metals suchas tungsten and molybdenum for the receptacle or holder considerabledifficulties arise in this respect.

According to the present invention the metal to be evaporated ordistilled on to the surface to be coated, and hereinafter called thecoating metal, is placed upon the surface of a heater preferably ofmolybdenum or tungsten and is brought 5 by heating in vacuo intointimate contact with the heater metal without being evaporated ordistilled and in such manner as to avoid the formation with the heatermetal of an alloy or mixture having a lower melting point than the evap-40 oration or distillation temperature of the coating metal, andthereafter the temperature is raised to cause the coating metal to beevaporated or distilled on to the relatively cool surface to be coated.For best results the vacuum should be 45 high, namely not less than ofthe order of 10- or 10- mm. pressure (mercury column) or lower.

The heater preferably comprises one 'or a plurality of strips ofmolybdenum or tungsten held between terminals to which electric heatingcur- 50 rent can be fed. For the avoidance of the formation of a lowmelting point alloy the coating metal conveniently can be applied uponsaid heater strip or strips in the form of thin uniform layers of finepowder namely in such small quantity as that any alloy formed has asmall percentage of (Cl. ill-70.1)

coating metal, for example not more than about 5 or 10% in the case forexample of aluminium on a molybdenum heater. In one particular example,0.1 gm. of fine aluminium powder was received upon a molybdenum heaterstrip about 5 5 inches long, about /;-inch wide and from 0.005

to 0.01 inch thick, this strip being heated first to a temperature whichwas estimated at about 900 C. This temperature was preferably maintaineduntil the alloying or mixing was completed 10 whereupon the evaporationor distillation of the aluminium was effected by raising the temperatureto what was estimated to be about 1800 (3., the coating being apparentlycompleted in about 1 minute. The surface to be coated was 4 inches 13 indiameter and was disposed about 12 or 18 inches above the heater'stripin the highly evacuated vessel the lower end of which was connected tocontinuously operating evacuating pumps maintaining a vacuum pressure of10- mm. withgo in the vessel.

For uniformly coating larger surfaces, it will be appreciated, aplurality of distributed heater strips may be employed whilst thesurface to be coated is preferably located at a greater distance 25 fromthe heater provided the vacuum can be maintained adequately high.

With the process according to the invention it is found that the heaterstrips do not become perforated for a considerable time so that although30 the strips become brittle they can be used many times particularly asit is unnecessary to remove them from their terminal clamps after eachcoating operation.

It will be understood that the surface to be coated must be thoroughlycleaned and that the heater strip and other internal parts, includingthe freshly applied powder should be thoroughly degassed by heatingand/or evacuating prior to the actual steps of the process of theinvention above set forth. The initial step is preferably carried outcomparatively slowly.

Coatings of alloys or mixtures can be deposited by employing two or moreheaters carrying respective different coating metals.

The process may be applied to the coating of continuous lengths of paperor fibrous material such as paper or textiles or silk provided theheaters are successively replaced such as by mounting a pluralitythereof on a rotating or travelling member.

Articles having thin translucent coatings may be used as light filtersby providing only a thin coating.

By the process of the invention beautiful coating transparent andprotecting the unoxidiaed.

coating beneath it against tarnishing. Thus articles or samples 0!special design for display, such as presentation cups and other silverware, may be coated with aluminium by the process or the presentinvention and displayed in show cases or shop windows for many monthswithout tarnishing and thus without any necessity for cleaning or.polishing. However the main application of the invention up to date isfor astronomical and epidiascope reflectors constituted by glass coatedwith aluminium.

The novel features which are characteristic of our invention. are setforth with particularity in the appended claims. The invention itselfhowever will best be understood from reference to the followingspecification when considered in connection with the accompanyingdrawing in which the single figure is a cross sectional view of theapparatus whereby my invention may be carried into effect.

Referring to the drawing i is the vessel in which the coating process iscarried out. Bald vessel is conveniently a hollow steel cylinder. Itrests upon a closure plate 2 having a central perforation 3, beneathwhich is welded a tube 4 connected to evacuating pumps (not shown) whichare preferably of the diifusion type employing a switch low vaporpressure oil. I'he plate 2 which is also conveniently of steel, restsupon a stand indicated at 5. The vessel is closed at the top by afurther plate 6, which is preierably provided with a central perforation6 closed by a glass window 8. Vacuum joints such as indicated at 9 maybe eflected by a suitable grease or wax, or at least the joint such asbetween the plate 6 and vessel i which is required to be made and brokenfrequently for the insertion of articles to be coated and of the coatingpowder, may be formed by a suitable wax or grease, whilst other Jointsmay be made more permanent such as by welding or by bitumen.

Fixed in the upper portion of the tube i and preferably adjustabletherein as to height is the support It for the member indicated at it tobe coated, which may be a glass disc ior in= stance.

The lower closure plate 2 is provided with at least two furtherperiorations through which pass the lead-in and support screws ii forthe heater strip l3 which is held at its end in clumps l4 secured to theupper ends of the members l2, which are, of course, insulated from theplate 2 through which they pass in a vacuum type arcane .ings may beobtained. particularly in the case manner such as may be obtained by theinsulating washers It. In practicing the method. the electric current isincreased until the coating metal powder appears to melt as may be seenthrough the window 8 and through the article ti (assuming the latter istransparent). This step should be carried out slowly, care being takennot to raise the temperature too high, as can be controlled by viewingthe glowing strip. Thus the temperature can be raised considerably untilthe coating metal boils oil the strip and deposits upon the article lluntil the coating is oi suiflcient thickness as can be controlled byobserving the reduction in transparency of the article;

whereupon the current may be switched-oil. It is found that the coatingsare fully annealed, and ii sumciently thick can be pealed of! thearticle.

It will be understood that the process hereinbefore set forth issusceptible to minor variations whilst the apparatus shown in thedrawing may be considerably modified, all within the scope of theinvention.

Having now particularly described and ascertained the nature of our saidinvention and in what manner the same is to be performed, we declarethat what we claim is:

1. The method of applying metal coatings to surfaces by evaporation ordistillation in a vacuum, which consists in placing the coating metalupon the surface of a metal heater, which is preferably electricallyheated molybdenum or tungsten, heating said coating metal to its meltingpoint in a vacuum to thereby bring it into intimate contact with theheater metal, the quantity of coating metal employed being so small thatany alloy formed with the heater element has a higher melting point thanthe evaporation or distillation temperature of the coating metal, andthereafter raising the temperature to cause the coating metal to beevaporated or distilled on to the relatively cool surface to be coated.

2. The method as claimed in claim 1, wherein the vacuum is maintained ata pressure 01 10- mm. mercury column or lower.

3. The method as claimed in claim 1, wherein the coating metal is placedon the heater surface in the form of thin uniform layers of powder.

4. The method as claimed in claim 1, wherein the heater is in the formof a strip.

5. The method as claimed in claim 1, wherein the heater is located inthe base of an evacuable vessel in which the article to be coated issupported above the heater.

CYRH. HUBERT

